Implement an alternative to the LXML backend

[Wuestengecko]

This could prove to be a viable alternative to #217. Initial experimentation has shown promising results with almost 50% reduction in memory use when using a specialized, custom parser built on top of quick-xml even in a still relatively naive implementation, where the main idea is to deduplicate tag and attribute names in memory (using a temporary HashMap to store Arc<(Option<String>, String)> over directly using the parsed-out (Option<String>, String) on the Rust side, but the same should hold true for the Python equivalents) — and there's probably still a lot of room for further optimizations.

This might still prove to be completely non-viable depending on the actual feature set that we'd have to reimplement, but so far the only bigger issues I've found are a few simple XPaths (which should be straight-forward to implement without an XPath engine) and the MelodyModel.find_references() function, which delegates to C code via an XPath query in order to actually become usable in terms of performance (if I'm not mistaken, runtime went down by at least 2-3 orders of magnitude).